The invention relates to a method and apparatuses for processing dosed quantities or doses of flowable material. In particular, the invention relates to a method and apparatuses to be used in the compression-moulding of dosed quantities of plastics, to obtain items such as preforms for containers, for example for bottles.
An aspect of the invention relates to a method and an apparatus for transferring or inserting dosed bodies in polymeric material in a more or less viscous liquid state dispensed by at least one dispensing port of polymeric material into the cavities of the mould of a moulding machine having a carousel that rotates with continuous motion, in compression-forming of items in polymeric material. Another aspect of the invention relates to a method and the corresponding means for handling bodies of polymeric material in a more or less viscous liquid state to be transferred to the mould cavities of a moulding machine, in compression-forming of plastics items.
Forming items by compression-moulding is achieved by moving a punch (the male element of a mould) relative to and inside of a hollow die (the female part of a mould). The punch is inserted through pressure inside the hollow die in which a dosed body of a more or less viscous liquid polymeric material has been deposited, in particular a thermoplastic resin. A particular application of the invention is for forming preforms intended for the subsequent manufacture (typically by stretch-blow moulding) of plastics bottles. Nevertheless, the applications may be different and varied.
The preforms for making bottles and the like usually comprise an upper neck provided with projections and a hollow body located below the neck, the hollow body being substantially smooth and axially elongated.
Typically, traditional moulding machines for manufacturing items in polymeric material by compression-forming comprise a carousel that carries a plurality of dies and a corresponding plurality of punches above. The carousel may rotate both continuously and intermittently.
The carousel rotates around a vertical axis and each die, during a revolution, receives a plastics body of a dosed quantity (dose) heated to the required temperature to make the plastics sufficiently fluid. The dose subsequently undergoes a pressing phase following the mutual approach (until closing of the mould) of the punch and the die. This phase is followed, after a given time, by the opening of the mould and the removal of the preform from the machine.
With the moulding machine an extruding device is associated that emits polymeric material in a more or less viscous liquid state. This material is divided into bodies of dosed quantity (doses) that are then transferred to the cavities of the dies of the machine.
If the dosed body (dose) of polymeric material has a relatively small mass, it is known to transfer it to the dies of the rotating machine by a transferring device having suitable removing members (so-called “hands”) that move in succession along a circular path. Along this path, the removing members pick up the dose from a fixed dispensing outlet belonging to the extruder of the polymeric material and release it at the point in which the path is tangential to and superimposed on the path of the dies.
This release must occur in a very fast manner at the instant at which the removing member is superimposed on the die cavity and coaxial with it. This is practically possible only in the case of doses of relatively small mass, for example doses suitable for forming capsule-shaped caps to close usual plastic bottles for mineral water or other sparkling drinks.
If on the other hand items in polymeric material have to be formed that have a relatively high mass such as preforms in PET (polyethyleneterephthalate) that are currently used on the market to produce (through the known stretch-blow moulding operation) the usual plastic bottles, loading the dosed body of polymeric material inside the dies is a very complicated operation.
In fact, in this case it is not practically possible to transfer the doses from the removing members through the die cavities with an action that occurs in an almost instantaneous manner because the doses have a relatively great length and sufficient time is thus required to perform this transfer. This time is not available in the disclosed traditional transfer devices.
To overcome this drawback, a moulding machine has been proposed (patent application WO 03/047834) in which the doses are emitted by a dispensing device having a plurality of dispensing outlets that are movable in succession along a circular and horizontal path. The dies do not move along a simple circular path but have a good possibility of moving in a radial direction in relation to the carousel and can therefore follow for a certain arc the further circular path of the dose dispensing outlets, deviating from the traditional circular path. Thus, for a certain portion of the path (and therefore for a certain time) the dispensing outlet is located coaxially and above the cavities of the die and its motion coincides with the motion of the latter.
Nevertheless, a drawback of the solution disclosed in WO 03/047834 is the complexity ad the constructional cost of the corresponding moulding machine. The machines according to WO 03/047834 are in fact very complex, both for the usually very high number of dies and for the numerous operations that are performed and the relatively high speed at which it is desirable that they operate. Lastly, the dies require very precise positioning and making them movable in relation to the carousel therefore complicates their positioning.
Furthermore, in the proposed solution, based on the radial movement of the dies, it is not even possible to achieve, in the shared portion, correct equality of the movements from the dies and of the dispensing outlets, which would nevertheless be necessary. In fact, the peripheral speed of the die varies as its radial position varies whereas this does not occur for the dispensing outlets, the radial position of which is constant. As a result, in the said shared portion the dispensing outlets cannot keep themselves coaxial with the die cavities below and thus the transfer of the dose cannot be correct; in fact, if the diameter of the dose is near the minimum diameter of the cavity it is not even possible to carry out the transfer.
The drawbacks disclosed above are exacerbated by the fact that the means with which the operations of transfer of the doses is performed, differing in form and nature, inevitably has surfaces that come into contact with such polymeric bodies in a liquid viscous state.
A further drawback of the prior art arises because the polymeric material of the doses tends to adhere to the surfaces of the means with which it comes into contact due to its physical state (more or less viscous liquid state at a temperature that is usually above 200° C., when it is PET).
The adhesive effect disclosed above inevitably hinders the movement of the polymeric body, creating serious drawbacks, especially if it is provided that the body simply moves through gravity. For example, if the polymeric body has to flow due to gravity along a surface that accompanies it, its tendency to adhere to the surface may hamper movement to the extent that the provided operation is made impossible.
Or, if the doses are dropped into the die cavities, the doses can adhere to the walls of the cavities, especially if the cavities have a relatively narrow and deep shape. If this occurs, the doses are unable to be positioned in a correct manner inside the die cavities. If, for example, the dosed polymeric body is of a relatively high volume in relation to the volume of the cavity there is a serious danger that the polymeric body will protrude above the cavity to such an extent that it is not possible to close the die during the phase of compression by the punch.
In particular during forming of preforms for bottles with a capacity of not less than a litre, the die cavity has a relatively narrow and elongated shape. There is therefore a relatively high risk that the descending dose will come into contact with the side wall of the cavity before reaching the bottom.
The drawbacks disclosed above of the adhesion of the dose to the surfaces with which it comes into contact are greatly exacerbated by the fact that the descent of the dose occurs whilst the die moves continuously along a circular path, and furthermore moves at a relatively high speed. In fact, owing to the centrifugal effect to which it is subjected, the polymeric body is pushed to the side wall of the cavity.
It should be noted that when doses of relatively small mass are transferred to the die cavity, the doses may undergo rolling or rotations, as they are provided with a substantially spherical form. When on the other hand the doses have a relatively high mass and a relatively complex shape, as occurs in the forming of PET preforms, it is normally necessary that the doses are normally placed in the mould cavity with their longitudinal axis arranged according to a preset orientation.
Furthermore, between the dose and the surfaces with which it comes into contact, an effective heat transmission occurs that is localised in the contact zone that consequently alters the regular and substantially uniform distribution of the thermal values of the polymeric body. In particular, excessive, albeit localised, temperature drops can easily be created that are such as to produce microcrystallisation or microsolidification of the polymeric material. Nucleuses of irregularity in the polymeric material are thus generated that may subsequently produce unevenness and defects in the final product.
Another drawback of the prior art is linked to the manner with which the removing members of the transferring device pick up the dose from the emission zone of the dispensing outlet.
Each removing member, is in fact generally provided with a concave contact surface, open on one side, suitable for impacting the dose immediately after it has been released by the dispensing outlet, pushing it with a horizontal component and guiding it in its descent to transfer it to the die cavity. Owing to the impact between the dose and the removing member, a rebound phenomenon of the dose on the contact surface nevertheless occurs. The dose may consequently be projected far from the removing member, or may, on the concave surface of the removing member, take up a position that is unsuitable for the subsequent descent.
In addition, cutting means suitable for cutting the outflow of polymeric material exiting from the dispensing outlet is normally associated with the transfer means, in such a way as to separate the dose in the instant immediately before the contact with the removing member. The cutting means may for example comprise a plurality of blades, each of which is fixed to a respective handling member.
During the cutting action, the blades subject the dose to a thrust with a horizontal component that, similarly to the aforementioned impact with the contact surface, can project the dose far away from the removing member, or cause poor positioning of the dose.
Also this defect is particularly significant if the dose has a significant mass and in particular has a form that is relatively very elongated.
A further defect of the prior art is that the known cutting means have rather a complicated structure. In fact, a number of blades has to be provided that is the same as the number of removing members, and each blade has to be correctly fitted to the removing member and sharpened or replaced when it is excessively worn.